In general, an OFDM technology has its applications in various digital data transmission systems such as a digital audio broadcasting (DAB), a digital television, a wireless local area network (WLAN), a wireless asynchronous transfer mode (WATM), etc. The OFDM is a multiple-carrier signal transmission technology where data modulated by a plurality of carrier signals are transmitted in parallel. Though conventional OFDM systems have not been widely used due to their structural complicities, recent development of various digital signal processing technologies such as a fast fourier transform (FFT) and an inverse FFT allows for commercial realizations of the OFDM systems in various fields. Though the OFDM method is similar to a conventional FDM method, its technical essence lies in that data transmission is performed while maintaining orthogonality between sub-carriers, thus obtaining optimum transmission efficiency for a high-speed data transmission. Accordingly, there have been suggested increasing number of data transmission technologies employing the OFDM method such as an OFDM/CDMA for use in a WATM system.
Referring to FIG. 1, there is provided a receiver of a conventional OFDM system.
The receiver of the OFDM system includes an A/D converter 10, a multiplier 12, a guard interval remover 14, a fast fourier transformer (FFT) 16, a frequency offset estimator 30 having a fine frequency offset estimator 18 and a coarse frequency offset estimator 20, and an adder 22.
The A/D converter 10 converts a baseband analog signal, received by a RF (radio frequency) receiver (not shown) and provided thereto, into a digital signal and then provides the digital data to the multiplier 12. The multiplier 12 compensates a frequency error included in the digital signal by using a predetermined frequency correction signal inputted from the adder 22, thus obtaining a sampled data signal.
Then, the guard interval remover 14 removes a guard interval from the signal outputted from the multiplier 12. To be more specific, the guard interval removing process involves the steps of: setting a window including two OFDM symbols and one guard interval; obtaining correlation values by moving the window on a sample basis on the outputted signal; defining a point in time when a maximum correlation value appears as a starting point of the guard interval; and removing from the outputted signal a data corresponding to the guard interval. The FFT 16 performs a fast fourier transformation on the data provided from the guard interval remover 14 to obtain stream type chip data. The chip data is sent to the frequency offset estimator 30.
Unless a transmitter and a receiver are synchronized during a local oscillating period in a data transmission system using the OFDM method, there may occur a frequency offset between the receiver and sub-carriers. If the frequency offset exists, interference between a received data and a neighboring channel may be incurred, so that the orthogonality between the sub-carriers may not be maintained. Thus, even a very minute offset can cause a deterioration of the efficiency of the receiving system. For this reason, the frequency offset estimator 30 is required in a data transmission system employing the OFDM method in order to compensate such a frequency offset.
The frequency offset estimator 30 calculates a correlation value between the chip data, provided from the FFT 16, and a preset reference signal and, then, outputs an estimated frequency offset. Specifically, the coarse frequency offset estimator 20 outputs an initial frequency offset corresponding to an integer multiple of the interval between carriers while the fine frequency offset estimator 18 outputs a residual frequency offset left after the first synchronization which is equal to or smaller than the integer multiple of the carrier interval.
The adder 22 receives the initial frequency offset and the residual frequency offset provided from the frequency offset estimator 30 and then outputs an estimated frequency offset, which is inputted to the multiplier 12 to serve as the frequency correction signal.
However, the conventional frequency offset estimator 30 as described above has a drawback as follows. In case a temporal domain method is employed to estimate the frequency offset, the detection range for the frequency offset is limited to a maximum of four times of the carrier interval though a small amount of calculation is involved. A frequency domain method, on the other hand, involves a great amount of calculation since all the possible correlation functions for the intervals between the sub-carriers should be calculated though there is no limit to the detection range for the frequency offset.
Assume that the transmitter (not shown) performs the transmission of the data by using N number of sub-carriers and the receiver receives two training symbols for the frequency synchronization in the OFDM system. At this time, the estimated frequency offset outputted from the frequency offset estimator 30 is obtained by using an algorithm to be described in detail hereinafter.
Referring to FIG. 3, there is illustrated a structure of a training symbol applied to the present invention. The training symbol includes guardtones and effective carrier signals. The guardtones serve to prevent the occurrence of noises due to interferences between neighboring training symbols.
The two training symbols X1, and X2, which are outputted from the transmitter, are defined as follows:
                              X          1                =                              ∑                          k              =              0                                      N              -              1                                ⁢                                    X              1                        ⁡                          (              k              )                                                          Eq.  1                                          X          2                =                              ∑                          k              =              0                                      N              -              1                                ⁢                                    X              2                        ⁡                          (              k              )                                                          Eq.  2            wherein N represents the size of a data to be FFT-transformed; k, and index indicating a sub-carrier component; and Xk, a complex data allotted to a k-th sub-carrier.
The two signals X1 and X2 are transmitted through a transmission channel and received at the receiver as two received signals, Y1 and Y2, respectively. The received signals Y1 and Y2 are represented as follows.Y1(k)=X1(k)H(k)+N(k)  Eq. 3wherein H(k) stands for a channel transfer function for the k-th sub-carrier and N(k) represents a noise component in the k-th sub-carrier. The offset estimator 30 obtains an estimated frequency offset from the received training signals Y1 and Y2 and compensates frequency offsets (foffset) of the received data signals by using an algorithm as follows.
                              f          offset                =                              1                          2              ⁢              π                                ⁢          arg          ⁢                                    ∑                              k                =                0                                            N                -                1                                      ⁢                                                            Y                  2                                ⁡                                  (                  k                  )                                            ⁢                                                Y                  1                  *                                ⁡                                  (                  k                  )                                                                                        Eq.  4            
However, in the offset estimation method using the frequency offset estimator 30 as described above, an excessively large amount of calculation is required since noise components generated by the guardtones, which have no transmission information, are also the subjects of calculation. Further, the offset estimation method using the frequency offset estimator 30 may accompany various problems such as an impulsive noise or a fading phenomenon caused by a frequency selection.